Purification of aryl esters of phosphoric acid



Patented Oct. 1 7, I933 UNITED No Drawing. Appl cation November 18, 1932 t gSeriaL No. 643,189

' olaimej ol. 260 -98) This invention relates to the purification of neutral phenolic esters of phosphoric and thiophosphoric acids, examples of which include tritolyl phosphate, triphenylphosphate, mixed 5 tolyl phosphates, tolyl xylyl phosphates, alkyl tolyl phosphates, alkyl phenyl phosphates, notably methyl diphenyl phosphate, ethyl ditolyl phosphate, etc. The present application is a continuation-in-part of my co-pending applications, Serial Numbers 485,317 and 558,337, filed September 29, 1930 and August 20, 1931, respectively. In my co-pending applications I have disclosed a method of purification which involves the use of potassium permanganate or other Water soluble permanganates at ordinary or slightly elevated temperatures with the view of improving the color, odor, and stability of the esters.

The phenols which areemployed in the manufacture of the esters, particularly the cresols and the xylenols are of coal tar origin and not infrequently include as impurities bases of the pyridine species as well as materials of a naphthalenic species, both of which, and the latter in particular, are objectionable because of the odor and other undesirable characteristics imparted to the final product. Since these impurities are present in the cresylic acids in very small amounts the separation therefrom is difiicult. At the same time the amount which is present is suflicient to impair the quality of the ester product.

I have now found that naphthalenic impurities may be removed conveniently after the ester isformed by subjecting the ester .to the action of an aqueous solution of a permanganate at elevated temperatures. The treatment may be effected either in an alkaline, neutral or acidic medium. In the former case an amount of soda ash is added to .the aqueous permanganate in order to maintain alkalinity during the course of the treatment. The separation of the precipitated manganese dioxide and other phases of the purification may follow that proposed in my aforementioned applications.

In practicing the invention as applied to tritolyl phosphate, which may include some xylyl phosphate esters by reason of the presence of xylenols in the cresylic acid utilized in preparing the ester, I wash the ester obtained by the condensation of phosphorous oxychloride and cresylic acid, with a caustic soda solution of 345% concentration in order to remove the alkaline soluble impurities, particularly the' acidic impurity, including the unreacted cresylic acid, the monotolyl and ditolyl acid phosphates, etc. This operationis performed advantageously at ordi-i nary room temperatures by agitating or otherwise washingthe ester with'the aqueous alkali solu.

'tion. It is convenient and advantageous. to wash the ester one, two or even three times, each time using afresh solution of the alkali. Thereafter the ester is washed with water to eliminate alkalinity and it is treated with a dilute aqueous solution of amineral acid such as'sulfuric acid while maintaining an elevated temperature preferably -100" 0., sufficient acid being employed to assure a distinctly. acid condition. For this purpose a Congo red indicator may be employed. In lieu of dilute sulfuric acid one may employ other acids; similarly one may employ other temperature conditions. For example, one may employ a small amount of concentrated hydrochloric acid at ordinary room temperatures. The acid wash treatment is continued or repeated, if necessary, until no pyridine-like odor is per ceptible.

Subsequently the ester is washed free of acid by means of water, preferably with the aidofa small amount of a base, such as soda ash. The resulting ester is then agitated with an equal or even larger volume of water containing approximately 2%, more or less, by weight of soda ash based on the Weightof the ester, to which potassium permanganate is added from time to "time to maintain a distinct purple coloration, the temperature being maintained at approximately -l00 C. As soon as the permanganate coloration is found to persist the excess permanganate is removed by washing with water and the heavy] precipitate of manganese dioxide is filtered or otherwise removed as by dissolving the same with.

a suitable reducing agent under acid conditions, 1

such as sodium sulfite or bisulfite or sulfurous acid. The esteris then'washed free of any water soluble substances and dried in the usual manner.

If desired, one may effect the permanganate treatment at elevated temperatures without the crude ester may be employed and other acid reacting or alkali reacting agents may be employed without departing from the spirit of this invention. Similarly the precise temperature conditions may be varied as wellas the time of rephate esters containing naphthalenic impurities,

the step which includes subjecting the ester to the action of an aqueous solution of a water soluble permanganate while maintaining a temperature sufiiciently high to decompose the naphthaienic impurities. v

2. In the purification of a neutral tolyl phosphate ester obtained by the inter-action of phosphorous oXychloride with cresylic acid of a coal tar origin containing naphthalenic impurities, the step in the purification of the ester whereby the naphthalenic impurities which comprises subjecting the'ester to the. action of an aqueous solutionof a water soluble permanganate While maintainingga temperature of approximately 80-100 C.

3. The method as defined in claim 2'and fur: therv characterized in that .the permanganate treatment is effected in the presence of an inorganic water soluble base.

"re eliminated 4. The method as defined in claim 1' and further characterized in that the permanganate treatment is continued until the characteristic permanganate: coloration persists after which precipitated manganese dioxide is removed from the ester by effecting dissolution thereof with the aid of an inorganic sulfite' reagentunder acid con 'ditions and in the presence of water.

same to the action of a sulfurous reducing agent in the presence of water. 7

7. In the purification of tri-aryl phosphate esters the step of washing the ester with :an

nate coloration persists. v

ERIK CLEMMEN'SEN.

- aqueous alkaline solution of a Water soluble per-- manganate until the characteristic permanga- 

